In dopamine primers 1 and 2, I introduced this useful neurotransmitter and key to the thinking style of humanity. Today we'll go over the different neurotransmitters on the different sides of the brain, and why that matters.
Dopamine is the neurotransmitter that is not only responsible for modulating a lot of our physical movement, but also sex, aggression, motivational drive, and, counter-intuitively, long-term planning and restraint or impulse control. In nearly all right-handed and most left-handed humans who are left-brain dominant, dopamine rules the left side of the brain. Lateral dopamine pathways modulate working memory, cognitive shifting, and other executive functions (such as planning).
On the right side of the brain - the artsy part - serotonin and norepinephrine are more dominant. Serotonin and norepinephrine have more to do with emotional activation and arousal systems. The serotonin systems manage our movements and focus on a close personal level. They work in close concert with the opiate reward system and some other hormonal and neuronal systems associated with close in work - feeding babies, eating - stuff we would do with our gaze directed downward, and usually doesn't require as much planning. Norepinephrine and serotonin networks also manage our vestibular systems, helping us balance, postural control, and knowing which way is up or down. Again, one associates balance with looking down or being centered in space. Norepinephrine tracts in the brain are also heavily involved in the "close-in" sense of touch.
The dopamine systems on the left side of the brain become more active with thinking and planning that is more long term. Scanning the horizon, searching and recognition. These systems rarely operate close in our personal space, as it is rare to bring something within arm's reach that we do not already recognize. The medial dopamine systems are used in exploration, navigation, and orientation to landmarks. Far vision, upward gaze, hearing, and smell are more associated with the dopamine tracts. Dopamine's linkage to distant space seems to have associated itself in humans with distant time as well. There is no species quite so future oriented as Homo sapien. And no other species has such high concentrations of dopamine in the brain.
An obvious and perhaps controversial correlate to these different neurotransmitter systems responsible for close/feeling/balance versus far/navigation/planning is the inherent female/male parallel. Aggressive "male" behaviors are mediated by dopamine, whereas receptive female and maternal behaviors (grooming, feeding) are mediated by close-in cues, norepinephrine, and opiate and oxytocin systems.
Psychiatrically speaking, it is probably not a coincidence that dopamine related disorders, such as schizophrenia, addiction, ADHD and even autism are much more common in men, whereas the serotonin/norepinephrine linked anxiety and depressive disorders are more common in women. Of course dopamine is also associated with depression and opiates with addiction, and men get depressed and anxious while women have ADHD and autism. These are obviously not absolutes, just trends.
It also occurs to me that the differences in personality disorder diagnoses between men and women may relate to the neurotransmitter dominance differences as well. Many more men have the diagnosis of antisocial personality disorder than women - basically, a sociopath. A person with antisocial personality disorder generally lacks empathy for others, and often struggles with impulsivity and aggressive drives and substance abuse. That combination will get you sociopathic behavior - stealing, murder, etc. Many more women than men are diagnosed with borderline personality disorder. Someone who has borderline personality generally has trouble with relationships and containing feelings appropriately. He or she can struggle with rage and impulsive behavior also, but often the violence is directed at him or herself in suicide attempts or self-injurious behavior. A person with borderline personality disorder has empathy but has trouble applying it - he or she likely has boundary issues and has a hard time sitting with another person's pain without feeling it too much as his or her own. Thus he or she will feel very deeply for someone, then when it gets to be too much, push him or her away. People with borderline also tend to have impulsive behavior and have higher risk for substance abuse. Both borderline and sociopathic individuals are likely to have experienced abuse or neglect as a child, but some have not and still have the disorder.
So if we run with the rather gross simplification that men are dopamine "left brain" dominant and women perhaps more balanced between the right and left hemispheres, you can see how troubles with regulating dopamine combined with an underactive social/empathy serotonin/norepinephrine system would get you sociopathy in a man. In a woman, the trouble in borderline personality seems to be more in the poor regulation of both - but perhaps more a deficiency of dopamine and a dysregulation of the social/empathy serotonin/norepinephrine side of things. Now these are all just my rampant speculations, but I'll keep my eye out for some actual proof.
And, of course, there are borderline men and antisocial women (think Sharon Stone in "Basic Instinct"). There is some question that some of our social male/female constructs keep us from wanting to diagnose women with antisocial personalities, and perhaps the same social constructs, pressures, and expectations mean that an abused boy child has the risk of growing up into a sociopath, and an abused girl child has the risk of becoming a borderline. I'm inclined to believe some of that is going on, but that the biology is also at play. Of course, many abused children grow up without having personality disorders as well.
I'll finish up with a discussion of how our brains end up the way they do in the first place. The same gravimetric serotonin-driven vestibular systems that help us stay balanced are responsible for the migration and localization of the dopamine tracts as they are formed in early development. The theory goes that our bipedalism keeps us oriented differently than other primates or other animals in utero, leading to different gravity signals and a more lateralized brain development (meaning our right and left brains are simply more different than the right and left brains of any other animals.). There are no known genes that act as master directors for this lateralization - it all seems to be related to epigenetic factors and serotonin availability in the right place at the right time.
Natural human movement, then, is what may make our brains human.
(once again I'm drawing from Previc's The Dopaminergic Mind in Human Evolution and History).
Also Previc's paper "The neuropsychology of 3-D space" http://www.ncbi.nlm.nih.gov/pubmed/9747184)